Differential thermal analysis cell assembly

ABSTRACT

This invention relates to a thermal analysis cell which is capable of supplying meaningful data in a high vacuum and which is particularly useful for operations within a mass spectrometer or similar instrument which allows the heating of samples within the confinement of the mass spectrometer vacuum or adjacent to the ion source. The cell comprises a silver block to guarantee uniform heat throughout the cell. It is insulated against the push-through shaft by a section of boron nitride, which is an excellent thermal insulator, and by sapphire or alumina, for example. The silver cell is equipped with two equal temperature sensing wells containing sample and reference temperature sensing means. The sample itself is loaded into a metal cup which is usually secured to its well, and which contacts the temperature sensing means. A similar cup, usually containing non-reactive material, is inserted in the reference well and contacts its temperature sensing means, usually thermocouple. The cell assembly has a detachable temperature sensing head part. External means are provided for utilizing the output of the reference temperature sensing element for use in controlling the heating of the cell.

United States Patent Langer et al.

[54] DIFFERENTIAL ANALYSIS CELL ASSEMBLY [73] Assignee: The Dow ChemicalCompany, Midland,

' Mich.

[22] Filed: Oct. 8, 1969 [21] Appl.No.: 864,657

Barrall et a1. D.T.A. Apparatus in'Analytical Chemistry Vol. 35 No. 12Nov. 1963, pp. 1837- 1840.

[4s] June 1972 Langer et Ial. Mass spectrometric D.T.A. in AnalyticalChemistry Vol. 37, No. 3 March l9 65 pp. 433and 434.

Primary Examiner-Richard C. Queisser Assistant Examiner-HerbertGoldstein A nomey-Griswold and Burdick and Earl D. Ayers [57] ABSTRACTThis invention relates to a thermal analysis cell which is capable ofsupplying meaningful data in a high vacuum and which is particularlyuseful for operations within a mass spectrometer or similar instrumentwhich allows the heating of samples within the confinement of the massspectrometer vacuum or adjacent to the ion source. The cell comprises asilver block to guarantee uniform heat throughout the cell. it isinsulated against the push-through shaft by a section of boron nitride,which is an excellent thermal insulator, andby sapphire or alumina, forexample. The silver cell is equipped with two equal temperature sensingwells containing sample and reference temperature sensing means. Thesample itself is loaded into a metal cup which is usually secured to itswell, and which contacts the temperature sensing means, A similar cup,usually containing non-reactive material, is inserted in the referencewell and contacts its temperature sensing means, usually thermocouple.The cell assembly has a detachable temperature sensing head part.External means are provided for utilizing the output of the referencetemperature sensing element for use in controlling the heating of thecell.

10 Claims, 3 Drawing Figures DIFFERENTIAL ANALYSIS CELL ASSEMBLYBACKGROUND OF THE INVENTION This invention relates to differentialthermal analysis'cells and particularly to differential thermal analysiscells for use in Y a high vacuum such as exists inside a massspectrometer, for example.

A thermal analysis cell which is capable of supplying meaningful data ina high vacuum is particularly useful for operations within a massspectrometer or similar instrument.

However, even though mass spectrometers are sometimes equipped withdevices which allow the heating of samples within the confinement of themassspectrometer vacuum orwithin the ion source, and such devicessometimes also allow the measurement of sample temperatures during theheating process, these devices do not allow the operation known asdifferential thermal analysis.

In differential thermal analysis, it is essential that the sample beheated at a predetermined rate of heating, usually a linear rate, thatthe sample temperature be known and indicated at all times, and fordifferential thermal analysis operations the sample temperature becontinuously compared with that of an inert material in the same cell.

In the past, this has required that three thermocouples located in thethennal analysis cell be precisely at the same temperature at all timesunless a chemical reaction occurs in the sample. Thus, it is also ofextreme importance that equal heat transfer is guaranteed from the heatsource to the thermal analysis cell, that no temperature gradient existsin the cell itself, that fast heat transfer is provided from the cell tothe sample and that each thermocouple remains electrically insulated.

In addition, to make a cell useful it must be possible to load a sampleinto the cell and introduce the cell with the sample into a massspectrometer without shutting down the operation of a mass spectrometeror other evacuated systems.

Further, it is desirable to be able to quickly change sample cellsand/or the sample cell head area.

A principal object of this invention is to provide an improveddifferential thermal analysis cell assembly.

Another object of this invention is to provide an improved simplifieddifferential thermal analysis cell assembly which is adapted to be usedin a high vacuum.

ln accordance with this invention, there is provided a cell assembly forinsertion in a high vacuum device. The cell assembly comprises closelybut detachably coupled blocks of good thermally conductivemetal, such assilver, for example, to guarantee uniform heat throughout the cell, anda pushthrough shaft part. lt is insulated against the push-through shaftby which it is inserted in the mass spectrometer or other high vacuumdevice by a section of boron nitride or other suitable material which isan excellent thermal insulator. The silver cell assembly is equippedwith two equal temperature sensing wells containing the samplethermocouple and the reference thermocouple, respectively. The sample isloaded into a metal cup, pushed into the sample well and secured theretoin contact with'its temperature sensing means. This temperature sensingmeans usually consists of a chromel-alurnel or other thermocouple whichcontacts the sample cup. The thermocouple and sample cup are usuallycemented in place in the silver block.

Means are provided whereby a furnace temperature control signal may bederived from the output of the reference cell temperature sensor.

Small sample sizes assure fast and uniform heating of the sample whilethe temperature sensor contacting the sample cup guarantees fastresponse to thermal effects in the sample itself.

The cell head and the rest of the cell body fit closely together, butmay be detached, permitting new heads to be interchanged with theremainder of the cell assembly.

The only efficient way of heat transfer in a vacuum is by radiation,thus the DTA cell itself is surrounded during operation by a radiativefurnace.

The invention, as well as additional objects and advantages thereof,will best be understood when the following detailed description is readin connection with the accompanying drawing, in which:

FIG. 1 is a side elevational view, partly broken away and in section, ofa difl'erential thermal analysis cell assembly in accordance with thisinvention;

FIG. 2 is a sectional view taken along the line 2-2 of FIG. 1, and

FIG. 3 is a simplified block diagrammatical view showing circuit meansfor deriving a reference signal and a heating furnace control signalfrom a single temperature sensor.

Referring to the drawing, there is shown a differential thermal analysiscell assembly, indicated generally by the numeral 10.

The assembly 10 comprises a cell head, indicated generally by thenumeral 12, a cell body part, indicated generally by the numeral 14, athermally insulating coupler 66 and a probe rod part 16.

The cell 12 comprises an elongated cylindrical silver block 20 having aflat end 74 and sloping rear end part 18 which contains axial off-centerbores 38, 40 which extends inwardly from the end 74 of the silver block20 and through the coupler 66 and rod part 16. The bores 38, 40 havesloping walled counter bores 82, 80, respectively at their ends whichcommunicate with the end 74 of the block 20.

Thermocouples 42, 44, in bores 38, 40, respectively, similarly havetheir cable leads 50, 34 extend to the connector 52 and suitableterminal pins 54.

Metal cups 46, 48 shaped to conform with the sloping counterbored parts82, of bores 38, 40, respectively, are fitted into the counterboredparts 82, 80, respectively and secured in place by suitable cement 60,62.

The thermocouple 44 is illustrated as having a fusion contact with thecup 48 (see filet 68) while a surface contact only is provided betweenthe cup 46 and thermocouple 42.

Because the cups are identical, either cup may be used as the samplecontaining cup. The cup used as the reference cup usually is filled withchemically non-reactive material such as particulated alumina, forexample, usually cemented in place.

The cell body section 14 is made of a material similar to the cell head,such as silver, for example, and is of generally cylindrical outerconfiguration.

The body section 14 has one end which has a large diameter slopingwalled bore 70 shaped to mate closely with the sloping walled end of thecell head 20. The cell head 20 is held in mating relationship with thebody part 14 by means of set screws 72,for example. r

The other end of the body part 14' is counterbored to provide aninternally threaded coupling part 84.

An electrical and thermally insulating coupling element 66 isconveniently made of boron nitride and is a solid cylindrically shapedelement having threaded outer wall surface which engages the threadedpart 84 of cell body section 14 and the threaded counterbored part 86 ofthe probe tube 16.

The probe tube 16 is of cylindrical configuration and has the cables 34,50 extending therethrough by means of the bores 38, 40 as previouslymentioned to terminate at one end to the connector 52 which is coupledto the end of the probe tube 16 which is most remote from the silverblock 18. In event the bores 38, 40 are not otherwise sealed, a seal isprovided near the connector end (52) of the probe tube 16.

A replaceable gas tight seal 90, 92 is provided between electrical leads34, 50 and bores 40, 38 at the end of the bores 40, 38, respectively,which are adjacent to the connector 52. This arrangement permits theeasy removal of the cell head 20 once the leads are disconnected fromthe connector 52 and the screws 72 are retracted. Also, this arrangementplaces the seals 90, 92 at the coolest part of the assembly.

An annular shaped handle and stop member 56 fits over the probe tube 16and is held in pre-determined position by the set screw 58. i

. gas tight thermally and electrically insulating outer seal between theprobe tube 16 and the cell body section 14 as the parts 16 and 14 aretightened on the coupler 66. The outer diameter of the sealing element65, made of sapphire or other suitable material, for example, is usuallythe same as the common outer diameter of thetube 16 and body section 14.

The entire assembly is adapted to be inserted into a high vacuum devicesuch as a mass spectrometer, for example through suitable sealable entrymeans (not shown). The sample and reference cups may be made of silveror other nonreactive metal which is an excellent thermal conductor. Acell probe entry and cell heating means which is especially useful inconnection with this and other cell assemblies is disclosed and claimedin F. J. Karles copending US. Pat. application Ser. No. 742,868,entitled Furnace Assembly for Thermal Analysis Use," filed July 5, 1968.

It has been found that the device satisfies the desired conditions thatthe temperature sensors (thermocouples) located in the thermal analysiscell should be precisely at the same temperature at all times unless achemical reaction occurs in the sample; that no temperature gradientexists in the cell itself, and that fast heat transfer is provided fromthe cell to the sample and that each thermocouple remains electricallyinsulated. The identical sample and reference cell cup assemblies helpassure that the only difi'erence in the temperature observed by theirrespective temperature sensors is due to characteristics of the samplematerial.

The use of a separate cell head and cell body section permits the quickchanging of cell heads for use with the rest of the assembly.

. Referring to FIG. 3, it may be seen that the outputs of the referencetemperature sensor 100 and sample temperature sensor 102 are coupled tothe input of a very high input impedance operational amplifier 104 in asubtractive relationship; The output of the amplifier 104 drives theabscissa of an X-Y type recorder 106.

The reference signal is also coupled to a very high input impedanceoperational amplifier 108 whose output is coupled to the input of veryhigh input impedance amplifiers 110, 112 respectively.

The output of the amplifier 112 is coupled to a heater control circuit114 which is in turn coupled to the furnace (not shown) used to heat thecell assembly.

The output of amplifier 110 is coupled to the ordinate drive of the X-Yrecorder 106.

The very high input impedance of 104, 108, prevents signal interactionwhich would hinder the use of the reference sensor signal also beingused as the furnace control signal (or vice versa). It is assumed thatthe placement of the second (usually the reference) temperature sensorin the assembly is such that the output signal is suitable as thefurnace control signaL, In addition, to make a cell useful it must bepossible to load a sample into the cell and introduce the cell with thesample into a mass spectrometer without shutting down the operation of amass spectrometer or other evacuated systems. This is easilyaccomplished with cell assemblies in accordance with this invention.

What is claimed is:

1. A differential thermal analysis cell assembly for use under highvacuum conditions, comprising a cell section having a removable cellhead part and a body part, the rear end of the being joined together inthe order listed in end to end relationship and having, when so joined,a generally cylindrical configuration with a smooth substantiallyconstant diameter outer surface, said cell section comprising agenerally cylindrically shaped part of highly thermally conductive metalhaving a sample cell receiving bore and a'reference cell receiving borein said head part which extends into said body part 2 each of said borescommunicating with a lar er diameter bore which extends inwardly fromthe rear end 0 said body part, each of said cell receiving bores havingsloping'walled counterbored parts at their forward ends, each of saidsloping walled counterbored parts having disposed therein a cup whosewalls mate with said sloping walled parts, said cups each beingthennally coupled to said cell head part, a pair of temperature sensingelements, one of said temperature sensing elements being disposed ineach of said cell receiving bores and each thermally contacting one ofsaidcups,.each of said temperature sensing elements having electricalleads which extend through said cell section, thermal isolation sectionand probe section, said cell section having coupling means adapted toreceive a part of said thermal isolation section, said thermal isolationsection comprising an element made of thermal and electrical insulatingmaterial which is coupled at its ends to said cell section and to saidprobe section, said temperature sensing elesensor in said reference cellreceiving bore into signals which are non-interacting with respect toeach other. i g

2. A cell assembly in accordance with claim 1, wherein said metal cupsare substantially identical. 8

3. A cell assembly in accordance with claim 1, wherein said means forsplitting the output of said temperature sensor comprises a pair of veryhigh' impedance amplifiers to whose inputs the output of saidtemperature sensor element thermally coupled to the cup used as areference is coupled in parallel.

4. Acell assembly in accordance with claim 1, wherein each of saidtemperature sensing elements is electrically insulated from the other.

5. A cell assembly in accordance with claim 1, wherein said part made ofhighly thermally conductive metal is made of silver.

6. A cell assembly in accordance with claim 1, wherein said thermalisolation section includes thermal and electrically insulating elementsealed between said cell section and said probe section, said annularelement having an outer diameter at least approximately the same as theouter diameters of the cell section and probe section.

7. A cell assembly in accordance with claim 1, wherein means areprovided for holding said head part and body part in operativepositional relationship.

8. A cell assembly in accordance with claim 1, wherein a gas tight sealis provided between said leads said bores extending through said bodypart extend through said thermal isolation section and said probesection and said leads extend therethrough and are sealed therein. I

9. A cell assembly in accordance with claim 1, wherein said cups arefusion sealed to said temperature sensing elements.

10. A cell assembly in accordance with claim 1, wherein said cupsmechanically contact said temperature sensing elements.

1. A differential thermal analysis cell assembly for use under highvacuum conditions, comprising a cell section having a removable cellhead part and a body part, the rear end of the head part mating with theforward end of said body part, a thermal isolation section and a probesection, said sections being joined together in the order listed in endto end relationship and having, when so joined, a generally cylindricalconfiguration with a smooth substantially constant diameter outersurface, said cell section comprising a generally cylindrically shapedpart of highly thermally conductive metal having a sample cell receivingbore and a reference cell receiving bore in said head part which extendsinto said body part 2 each of said bores communicating with a largerdiameter bore which extends inwardly from the rear end of said bodypart, each of said cell receiving bores having sloping walledcounterbored parts at their forward ends, each of said sloping walledcounterbored parts having disposed therein a cup whose walls mate withsaid sloping walled parts, said cups each being thermally coupled tosaid cell head part, a pair of temperature sensing elements, one of saidtemperature sensing elements being disposed in each of said cellreceiving bores and each thermally contacting one of said cups, each ofsaid temperature sensing elements having electrical leads which extendthrough said cell section, thermal isolation section and probe section,said cell section having coupling means adapted to receive a part ofsaid thermal isolation section, said thermal isolation sectioncomprising an element made of thermal and electrical insulating materialwhich is coupled at its ends to said cell section and to said probesection, said temperature sensing elements being differentially coupledto provide an electrical signal which is a function of the temperaturesexisting in the cups, and means for splitting the output of thetemperature sensor in said reference cell receiving bore into signalswhich are non-interacting with respect to each other.
 2. A cell assemblyin accordance with claim 1, wherein said metal cups are substantiallyidentical.
 3. A cell assembly in accordance with claim 1, wherein saidmeans for splitting the output of said temperature sensor comprises apair of very high impedance amplifiers to whose inputs the output ofsaid temperature sensor element thermally coupled to the cup used as areference is coupled in parallel.
 4. A cell assembly in accordance withclaim 1, wherein each of said temperature sensing elements iselectrically insulated from the other.
 5. A cell assembly in accordancewith claim 1, wherein said part made of highly thermally conductivemetal is made of silver.
 6. A cell assembly in accordance with claim 1,wherein said thermal isolation section includes thermal and electricallyinsulating element sealed between said cell section and said probesection, said annular element having an outer diameter at leastapproximately the same as the outer diameters of the cell section andprobe section.
 7. A cell assembly in accordance with claim 1, whereinmeans are provided for holding said head part and body part in operativepositional relationship.
 8. A cell assembly in accordance with claim 1,wherein a gas tight seal is provided between said leads said boresextending through said body part extend through said thermal isolationsection and said probe section and said leads extend therethrough andare sealed therein.
 9. A cell assembly in accordance with claim 1,wherein said cups are fusion sealed to said temperature sensingelements.
 10. A cell assembly in accordance with claim 1, wherein saidcups mechanically contact said temperature sensing elements.